1. Field of the Invention
The present invention relates generally to water treatment. More particularly, the present invention relates to a system and method for removing ammonia from water, such as wastewater.
2. Related Art
Wastewater treatment lagoons are one of the most widespread treatment technologies in the United States, and quite possibly one of the most neglected. Lagoons as a treatment technology are suited for small to medium sized rural communities, animal feedlot operations, as well as some industries. The primary advantages of lagoons are low cost and ease of operation. Generally speaking, lagoons are effective at removing organic material and suspended solids, provided the lagoons are not overloaded. One disadvantage of most types of lagoon systems is their inability to remove ammonia compounds from the water.
Ammonia is the primary cause of stench and subsequent neighbor complaints from lagoon systems. Ammonia is not removed from the wastewater stream in lagoons because the growth of nitrifying bacteria is not encouraged. These bacteria are inhibited by sunlight, and are out-competed by algae and most other free-floating bacteria.
The lack of nitrification of ammonia compounds is due to several factors inherent in the design of an open lagoon. The conversion of ammonia to nitrite and then nitrate depends on a class of bacteria known collectively as nitrifiers. Nitrifying bacteria are somewhat fickle compared to other bacteria such as zooglea and organisms like algae that thrive in wastewater treatment lagoons. Nitrifying bacteria are slower growing than zooglea and algae. They are also inhibited by direct sunlight. They have a total oxygen demand to convert ammonia to nitrate that is quite high. These bacteria are also temperature sensitive, and are generally inhibited at temperatures below 11° C. It is also known that the waste secretions from certain strains of algae can be inhibitory to nitrifying bacteria.
Aerobic and anaerobic decomposition of nitrogenous organic compounds in the lagoon release ammonia into the water column, thus adding to the dissolved ammonia levels. The TKN (Total Kjeldahl Nitrogen) level of the influent wastewater can be thought of as an indicator of the ultimate possible ammonia loading as the nitrogen in the organic compounds is biologically converted to ammonia. One additional source of ammonia in the water should be mentioned. A few genera of photosynthetic algae can also fix atmospheric N2 gas (i.e. convert N2 into NH3). The extent of ammonia addition to wastewater treatment lagoons has not been quantified at this time.
At neutral pH levels, the ammonia molecule is in the form of ammonium (NH4+), a highly soluble compound with a low water-to-air transfer coefficient. In other words, ammonium wants to stay in solution with the water. Gas stripping of ammonia is usually accomplished by adjusting the pH of the water to around 10.5. An example of this is given in Tchobanoglous, G. and E. D. Schroeder, Water Quality, 535–538 (Addison-Wesley, 1987).
Trickling filters are one of the oldest forms of wastewater treatment. Rocks, or other media designed to have a high surface area to volume ratio are stacked in a basin and wastewater is trickled over the media. Attached growth organisms metabolize the organic material out of the wastewater as it flows past the surface. The thickness of the film provides conditions suitable for aerobic bacteria at the free surface, and anaerobic bacteria near the media surface. Nitrifying and denitrifying bacteria are considered facultative bacteria and thrive in the interface between the aerobic and anaerobic zones. Trickling filters are effective at removing ammonia from wastewater due to this extensive zone favorable to the growth of ammonia consuming organisms. Growth of these organisms is favored because the sunlight is blocked in the depths of the filter, the metabolism of the bio-film increases the temperature within the bio-film, the fixed media provides extremely long detention times for the bacteria (the film remains in place until it becomes so thick that it sluffs off), and the exact oxygen requirements for the nitrifying bacteria and the denitrifying bacteria will be met at some point across the thickness of the bio-film.
Other designs that provide surface area for fixed film growth are Rotating Biological Contractors (RBCs), and various designs that place foam blocks and spacers or fibrous material down in the wastewater.
The primary disadvantages of a trickling filter are the initial capital costs to build the filter, pumping costs to lift the wastewater plus recycle to the top of the filter, maintenance of the mechanical distribution system at the top of the filter, and ultimate disposal replacement of the media within the filter. (Plastic media within the filter has an estimated life of 10 to 15 years, and must be disposed of as a hazardous waste when removed.)
RBCs require mechanical rotation systems, and provide much less surface area than plastic media filters. Capital costs to reach the equivalent surface area of a trickling filter can be quite high, although the energy costs to rotate the devices are generally a fraction of the pumping costs for trickling filters.
The metabolism of nitrifying bacteria is enhanced when the bacteria are immobilized on a fixed film surface, as opposed to free-floating bacterial colonies. Scandinavian researchers subjected the species Nitrobacter agilis to temperature variations from 30° C. to 12° C. Suspended growth bacteria experienced a 90% reduction in nitrification activity, whereas the fixed film bacteria only experienced a 20% reduction in nitrification activity. Other advantages of attached growth bacteria are enumerated in an article by Criddle et al. found in Bear, J. and M. Y. Corapcioglu, eds., Transport Processes in Porous Media, 641–691 (Kluwer Academic Pubs, 1991).
Most lagoon systems are devoid of oxygenated surfaces that are blocked from the sunlight. The bottom of the lagoon does not provide surface area because it is unconsolidated media and is anoxic. As such, lagoon systems do not nitrify ammonia compounds. High ammonia levels are fairly typical in lagoon effluents.